Acquiring sharply-focused images of moving people or objects is a fundamental and challenging problem in several applications. In many applications, but particularly in iris and iris image capture, subject/target motion is a complicating factor. In moderate lighting environments (e.g. indoors), the long exposure times required to generate a clear image limits the range of subject velocities that can be tolerated. For higher subject velocities, motion blur in the captured image may prevent exploitation, e.g. iris recognition.
For applications such as iris recognition, wherein fine scale features are essential to proper classification, the use of a traditional shutter imposes some fundamental limits on the extent of motion blur that can be tolerated. Motion blur, as through a traditional shutter, is equivalent to convolution of a sharply-focused image with a box filter. Motion-blurred images of this type lack information regarding the object at a number of spatial frequencies. This lack of information is irreversible and no post processing can recover it from the image. Methods that attempt to deblur the image will severely amplify sensor noise, hallucinate content, or both. Though it may be useful in improving subjective image quality, hallucinating image content is counter-productive for forensic applications, and amplifying noise complicates iris matching.
To avoid this loss of information during image capture, some prior art approaches have advocated the use of a fluttering shutter and demonstrated the ability to recover high-quality images despite blur from moving objects. During exposure, the camera's shutter flutters between open and closed while exposure is accumulated. This produces an image with coded blur which, unlike traditional blur, conveys information about the subject at all spatial frequencies. Given a suitably designed processing method that is based on the shutter's fluttering pattern, deblurring recovers an image with low levels of noise while avoiding reconstruction artifacts.
Researchers have developed methods to mitigate motion blur post-capture, including the flutter shutter technique. Note that a non-limiting example of a flutter shutter camera and flutter shutter technology in general is disclosed in U.S. Patent Application Publication No. US2007/0258707A1, entitled “Method and Apparatus for Deblurring Images,”which published to Ramesh Raskar on Nov. 8, 2007 and is incorporated herein b reference. Another non-limiting example of a flutter shutter camera and flutter shutter technology is disclosed in U.S. Patent Application Publication No. US2007/0258706A1, entitled “Method for Deblurring Images Using Optimized Temporal Coding Patterns,” which published to Ramesh Raskar, et al on Nov. 8, 2007, and is incorporated herein by reference.
There is a need to provide improved systems and methods that are capable of acquiring sharp images of subjects despite subject motion.